It's not just doing a single extra shift though. It is because of management incompetence around resourcing it's likely to be at least one additional shift every week. And at some point, probably quite quickly, the value of spending time with family and friends becomes more important than what will be less than £300 after tax.
And this takes into no account of how low morale is because of the company stalling on any pay increases for 5 years and counting.
I know my goodwill to help out would be in short supply with any role under when the above situation applies.